1. Field of the Invention
The present invention relates to image sensor array technology for use in electronic cameras employing solid state pixel sensor arrays. More particularly, the present invention relates to image scanning techniques for displaying a high-resolution image sensor pixel array onto a viewscreen of lower resolution.
2. The Prior Art
Active pixel sensor arrays are well known in the prior art. Readout techniques for active pixel sensor arrays combine pixel sensor addressing schemes with scanning circuitry suitable for reading solid state sensor arrays. Exemplary scanning circuitry suitable for vertical and horizontal scanning is described in Analog VLSI and Neural Systems, by Carver A. Mead, Addison Wesley Publishing Co., 1989, at pp. 263-267.
Pictures from active pixel sensor arrays are often displayed onto viewscreens which have a lower resolution than the active pixel sensor array itself does. In order to fit the picture onto the viewscreen, some pixels are not displayed. Two known methods of skipping some of the pixel sensors in the array are panning and zooming. Panning involves changing the start location of readout along the vertical or horizontal active pixel sensor array axis. Zooming involves mapping an image of higher resolution onto a viewscreen of lower resolution. Such zooming is accomplished by selectively displaying only a portion of the total rows and columns of active pixel sensor array data.
The prior art pixel sensor addressing schemes employed by scanning circuitry for active pixel sensor arrays are generally capable of either the random access of individual pixel sensors, or the sequential access of pixel sensors. The random accessing of pixel sensors allows for the readout of any individual pixel sensor in the active pixel sensor array. Sequential access allows for the readout of all of the pixel sensors in an active pixel sensor array, where each frame is scanned out one line at a time.
Some prior art imaging systems read out the entire active pixel sensor array and store the image data in memory. Subsequent manipulations are performed on the memory to access selected portions of the image data for display onto a viewscreen of lower resolution. Such imaging systems necessitate reading out the entire active pixel sensor array regardless of what image data is actually desired.
Other prior art imaging systems use off-chip processing circuitry to determine which portions of pixel sensor data are desired for display. The entire active pixel sensor array is read out sequentially. While readout is occurring, a timing controller selectively transfers only the pixel sensor data desired to the viewscreen display. Such imaging systems also necessitate reading out the entire active pixel sensor array.
Still other prior art imaging systems can be adjusted to change the number of pixel sensors read out at a given time. Such imaging systems are able to read out multiple pixel sensors simultaneously to produce a single average value. This technique increases the scanning speed of the imaging system while decreasing the resolution of the sensor array data. However, such systems involve additional circuitry to average the pixel sensor values.
In prior-art systems using random-access imager arrays, external logic could be used to select rows and columns for pan and zoom, but this requires high-speed address busses and added logic which may be a speed bottleneck.
In the art of color electronic imaging, it is necessary to resolve the incoming image into three color channels. Prior art active pixel sensor arrays are typically combined with a color separation prism, which splits the incoming image into three separate images, one in each wavelength range. Three separate active pixel sensor arrays are used in conjunction with the color separation prism, each capturing the image from one wavelength range. When the image data from the three separate active pixel sensor arrays are recombined, they form the basis of a color image.
Prior art color separation prisms such as that disclosed in U.S. Pat. No. 4,084,180, issued Apr. 11, 1978 to Stoffels et al., resolve an incoming image into three separate color beams. All beams exiting the color separation prism have the same image orientation, and thus none of the three-color separation beams are mirrored compared to the other color beams. However, this type of color separation prism has the disadvantage of requiring an air gap within the prism itself to achieve the same image orientation for all three-color separation beams. Such prisms can be difficult to manufacture properly.
A second type of color separation prism, such as that disclosed in U.S. Pat. No. 4,072,405, issued Feb. 7, 1978 to Ozeki, avoids the necessity of having an air gap within the prism. However, this color separation prism produces three different color beams wherein one beam""s orientation is a mirror image as compared to the other two color beams"" orientations. The active pixel sensor array receiving this mirrored color beam should be able to reverse its image readout direction to match the image orientation of the other two color beams.
It is an object of the present invention to provide scanning circuitry for an active pixel sensor array with row and column addressability that can combine the features of an arbitrary pixel sensor readout starting location with a sequential pixel sensor addressing mode. Different scanning modes with different image resolutions are achieved by counting pixel sensor rows in steps of Kn for row addressability and counting pixel sensor columns in steps of Km for column addressability, allowing for selective pixel sensor skipping. Selective pixel sensor skipping allows for increased scanning speed and decreased image resolution, producing an image suitable for viewing on a low-resolution viewscreen.
It is another object of the present invention to provide on-chip scanning logic for row and column addressing to perform sequential pixel sensor addressing with pan and zoom modes.
Yet another object of the present invention is to provide easy panning control, which changes the start readout location along the vertical and horizontal dimensions of the active pixel sensor array. Each sequential pixel sensor addressing mode allows for an arbitrary pixel sensor readout starting point.
Yet another object of the present invention is to provide a means for storing several values of Kn and Km on-chip, to provide for several different image resolutions.
Yet another object of the present invention is to provide a means for selectively complementing the row or column pixel sensor address counting sequence during pixel sensor selection. This feature is useful when the active pixel sensor arrays are used with a color-separation prism whereby some prism outputs may be mirrored. The pixel sensor address complementing feature returns the mirrored prism output to the standard output orientation. The complementing feature is available for both the row and column pixel sensor selection. This allows the active pixel sensor arrays receiving each color separation beam to be placed in any orientation (all vertical or all horizontal, with 180-degree rotation being allowed).
The present invention describes an apparatus and a method for scanning an active pixel sensor array that provides multiple pixel sensor addressing modes. The apparatus and method are used in conjunction with a solid state active pixel sensor array for use in either a still or video digital camera. A preferred active pixel sensor array with which the present invention may be used comprises a plurality of integrating photosensors placed in rows and columns. Each photosensor on a row is connected to a common row select line. Each photosensor on a column is connected to a common column output line. Illustrative different types of active pixel sensor arrays suitable for use with the scanning apparatus disclosed herein are described in U.S. Pat. Application Ser. No. 08/969,383, xe2x80x9cINTRA-PIXEL FRAME STORAGE ELEMENT, ARRAY AND ELECTRONIC SHUTTER METHOD SUITABLE FOR ELECTRONIC STILL CAMERA APPLICATIONSxe2x80x9d, to Merrill et al filed on Nov. 13, 1997, U.S. Pat. Application Ser. No. 09/031,333, xe2x80x9cEXPOSURE CONTROL IN ELECTRONIC CAMERAS BY DETECTING OVERFLOW FROM ACTIVE PIXELSxe2x80x9d, to Merrill et al., filed on Feb. 26, 1998, although the present invention will be advantageous when used with other types of pixel storage elements and arrays.
The image scanning apparatus described herein is capable of performing both high-resolution scanning modes and low-resolution scanning modes. The user is able to arbitrarily select the starting point for the image scan, and also to select a mode in which to view the image. The availability of different image viewing resolutions is useful when the active pixel sensor array is a high-resolution array with N rows and M columns of pixel sensors, and the viewscreen used for display has a lower resolution and fewer rows and columns for pixel display. It is not possible to display all pixels from the high-resolution pixel array onto the viewscreen, and a method for selecting which pixels to display is necessary.
High-resolution full image capture occurs when the scanning circuitry reads out all of the pixel sensors sequentially from the entire active pixel sensor array. This full image capture mode of readout is too large to fully display onto the low-resolution viewscreen.
High-resolution partial image display mode occurs when only a selected portion of the total pixel array is displayed onto the viewscreen. The user selects an arbitrary individual pixel sensor for beginning readout. The image scanning apparatus is capable of accessing any starting location within the active pixel sensor array. This allows image scanning to begin at any selected location within the pixel sensor array. The image scan will proceed sequentially from the initial address, displaying each pixel from the pixel sensor array onto the viewscreen until a predetermined number of rows and columns has been displayed. This arbitrary start point addressing feature allows a selected portion of the high-resolution pixel sensor array to be displayed onto a viewscreen of lower resolution, without sacrificing any detail.
The full frame viewscreen display mode allows the entire high-resolution N rows by M columns pixel sensor array to be displayed onto a viewscreen of lower resolution. The full frame viewscreen mode, by counting in steps of Kn for row addressability and counting in steps of Km for column addressability, selectively skips rows and columns of the active pixel sensor array when selecting pixel sensors to display onto the viewscreen. This selective skipping allows the higher-resolution pixel array to be mapped onto the lower-resolution viewscreen. Selective pixel sensor skipping will cause some image detail to be lost, but the user will be able to see a lower-resolution version of the entire image.
In the full frame viewscreen mode, Kn and Km may be determined by finding the smallest whole values of Kn and Km that will allow the full pixel sensor array to be mapped onto the viewscreen. To maintain the aspect ratio of the image, it is typically preferable to use equal values of K (Kn=Km) in the two dimensions, even though the invention and the embodiment disclosed herein will support different values of Kn and Km. For example, if the viewscreen can only display Vr rows and Vc columns of pixels:
K=ceiling (max (N/Vr, M/Vc))
where the ceiling function means the least integer that is equal to or greater than its argument.
Additional medium zoom modes of display are also available. Different values of Kn and Km may be selected, allowing for different resolution display modes. Several Kn and Km storage registers are provided in the scanning circuitry, allowing different values of Kn and Km to be selected by the user. This makes multiple resolution levels available. For the additional medium zoom modes of display, the user may arbitrarily select the starting location.
The scanning apparatus is thus capable of displaying several different viewing modes onto a viewscreen. In high-resolution full image capture, the entire active pixel sensor array is read out. The entire image will not fit onto the viewscreen in this mode. In high-resolution partial image display mode, the user may choose an arbitrary starting point from which the scanner will sequentially display the image from the pixel sensor array onto the viewscreen. No pixel sensors within the displayed region are skipped in this mode, and generally not all of the pixels will be displayed onto the viewscreen. In the full frame viewscreen display mode, the entire image from the pixel sensor array will be displayed onto the viewscreen. In order to accomplish this, it is necessary to selectively skip pixel sensors in order to display the full image. Several medium zoom modes are also available between the two extremes of high-resolution full image capture mode and full frame viewscreen display mode.
It is envisioned that a user would first choose to view the image from the pixel sensor array in the low-resolution full frame viewscreen display mode to get an overall view of the image. The user could then select a smaller area of the image for viewing in one of the medium zoom modes, eventually proceeding to high-resolution partial image display mode for viewing the smallest area of interest within the image.
The scanning circuitry disclosed herein uses row and column address counters to determine the addresses of pixel sensors to be read out from the active pixel sensor array. The row and column address counters provide pixel sensor addresses to the row decoder and column selector, which in turn perform the actual pixel sensor readout. The selective row and column skipping schemes described above for the various pixel sensor selection modes of the scanning circuitry are implemented using values of Kn and Km stored within the row and column address counters.
Complementors are also included in each row and column address counter, allowing for image mirroring in both the X and Y directions. The use of a color-separation prism within the camera to resolve the incoming image into three color channels means that one or more of the color separation beams may have an orientation that is mirrored with respect to the other color separation beams. The X and Y complementors allow the orientation of a particular color separation image to be reversed, putting it back into the correct orientation to be recombined with the other color separation beams.
The X or Y complementors will, if activated, reverse the scanning order for either the rows or the columns. Thus, instead of starting at row zero and progressing to row Kn, the row-address counter will instruct the row decoder to start reading pixel sensors at the opposite edge of the pixel sensor matrix (2047, for example) and count down by Kn to 2047-Kn. The complementors are controlled by a bit indicating the desired image parity in each direction. If the active pixel sensor array in question receives a color separation beam with a mirrored orientation, the complementor feature can be activated for the scanning circuitry associated with that particular active pixel sensor array. The color-separation prism will require one dimension to be complemented, depending on the actual spatial orientation of the active pixel sensor arrays and the orientation of the color-separation prism itself.